We describe a new library, marching.js, designed for ray marching and constructive solid geometry in the browser. The API was designed to be easy to use for live coding while enabling volumetric rendering techniques—inspired by the demoscene—that would be difficult to achieve via the rasterization of geometries or by writing fragment shaders from scratch. The library and an associated live coding environment enable programmers to easily experiment with these techniques without needing to know details about the underlying ray marching or lighting algorithms.